Measuring 3D profiles of articles is commonly used in the field of postal sorting for the purpose of obtaining information on size, height, and orientation of parcels being conveyed flat on a conveyor.
That is why certain postal sorting machines are equipped both with a sensor installed along the conveyor and capable of taking scans of the articles that are moving along it, and also with a data processor unit suitable for retrieving the scans from the sensor so as to reconstruct a 3D profile of the moving article. Presently, several techniques are commonly used in postal sorting, each having its own specific features.
For example, stereoscopy makes it possible to reconstruct the relief of an article on a conveyor on the basis of two images showing the article flat on the conveyor taken from two distinct angles. An analysis of the difference in the positions of characteristic points between the two images makes it possible to compute a position in depth for each of said points relative to a fixed point, and thus to determine the 3D profile of the article.
However, that technique suffers from the risk that the article to be measured might have few characteristic points, in particular when zones are present that are uniform between the article and conveyor, leading to poor matching between the points on the two images and thus to an erroneous measurement.
Laser triangulation is also widely used because it makes it possible to achieve very good accuracy for the height measurement with measurement distances as great as several meters, thereby making it possible to cover the spectrum required for postal sorting. Laser triangulation operates with a camera that measures the deformation of a laser line projected at a known angle onto the surface to be measured. French Patent Document FR 2 929 481 discloses a method of performing visiometric inspection on baked products that uses laser triangulation.
However, the measurement is limited in the conveying direction in which the article is conveyed, and that limitation generates blind spot zones that are detrimental when the articles have complex shapes or when articles are superposed on one another.
Other techniques are also used in fields other than postal sorting, such as “Time Of Flight” (TOF) that consists in illuminating the conveyor and the articles with a flash of light and in computing the time it takes for the flash to travel from the article to the optical sensor. The time of flight of that flash is directly proportional to the distance between the optical sensor and the measured article. The time of flight is measured independently by each pixel of the optical sensor, thereby making it possible to obtain a full 3D image of the measured article.
However, the long integration times of TOF cameras are inappropriate for making it possible to obtain total acquisition of an article moving on a conveyor. In addition, that technology does not propose sufficient resolution for the measurement accuracy desired for postal sorting.
Unfortunately, in postal sorting, it is very frequent for parcels to be conveyed loosely (superposed, juxtaposed, tipped on edge, etc.). None of the above-presented techniques make it possible to measure the 3D profiles of such moving parcels correctly.